Handle annotations not available at runtime (#941)

* Add missing runtime annotations red test case

* Update red test to use field_to_swagger_object

* Add graceful fallback for get_type_hints NameError exceptions

* Fix whitespace linter errors

* Bump cmarkgfm

* Bump cmarkgfm

* Add missing commas

Author: JoelLefkowitz

requirements/docs.txt

@@ -4,6 +4,6 @@ sphinx_rtd_theme>=0.2.4
 Pillow>=4.3.0
 readme_renderer[md]>=24.0
 twine>=1.12.1
-
+cmarkgfm>=0.8.0,<0.9.0
 Django>=2.0
 djangorestframework_camel_case>=0.2.0

src/drf_yasg/inspectors/field.py

@@ -7,6 +7,7 @@
 from collections import OrderedDict
 from contextlib import suppress
 from decimal import Decimal
+from inspect import signature as inspect_signature
 
 from django.core import validators
 from django.db import models
@@ -709,8 +710,22 @@ def field_to_swagger_object(
                 serializer, swagger_object_type, use_references, read_only=True
             )
         else:
-            # look for Python 3.5+ style type hinting of the return value
-            hint_class = typing.get_type_hints(method).get("return")
+            try:
+                # look for Python 3.5+ style type hinting of the return value
+                hint_class = typing.get_type_hints(method).get("return")
+
+            except NameError:
+                hint_class = inspect_signature(method).return_annotation
+
+                if hint_class is not None and hint_class != inspect._empty:
+                    SwaggerType, _ = self._get_partial_types(
+                        field, swagger_object_type, use_references, **kwargs
+                    )
+
+                    return SwaggerType(
+                        type=openapi.TYPE_STRING,
+                        description=f"Return type: {hint_class}",
+                    )
 
             # annotations such as typing.Optional have an __instancecheck__
             # hook and will not look like classes, but `issubclass` needs

tests/test_annotations.py

@@ -0,0 +1,35 @@
+from typing import TYPE_CHECKING
+
+from rest_framework import serializers
+
+from drf_yasg import openapi
+from drf_yasg.inspectors.field import SerializerMethodFieldInspector
+from drf_yasg.openapi import ReferenceResolver
+
+if TYPE_CHECKING:
+    from uuid import UUID
+
+
+def test_missing_runtime_annotations():
+    class UserSerializer(serializers.Serializer):
+        uuid = serializers.SerializerMethodField()
+
+        def get_uuid(self, obj) -> "UUID":
+            return obj.uuid
+
+    field = UserSerializer().fields["uuid"]
+    components = ReferenceResolver("definitions", "parameters", force_init=True)
+
+    inspector = SerializerMethodFieldInspector(
+        view=None,
+        path="/",
+        method="GET",
+        components=components,
+        request=None,
+        field_inspectors=[],
+    )
+
+    schema = inspector.field_to_swagger_object(field, openapi.Schema, True)
+
+    assert schema.type == "string"
+    assert schema.description == "Return type: UUID"